Just be prepered for hill after hill after hill. Don't know if you are from the flats or the hill country but if you are not prepared for MANY short ascents you will become discouraged.
I second the idea of staying on one island and taking the ferries to the other islands and day tripping. Orcas has the busiest road into town as that is about the only one everybody will drive from the ferry.
Be sure to walk all the way into and South in Shark Reef Park on Lopez to see seals basking on rocks. Make sure to stop in Lime Kiln Park on San Juan. If you get very lucky you will see an Orca pod swim by just yards off shore.
To lessen the overall biking you can put your bikes on the car, drive from your room or campground and park for the day near the ferry. Then all you have to do is roll your bike onto the boat.